1. Field of the Invention
This invention relates generally to microfluidic devices and systems. More particularly, it relates to a novel electrokinetic instability micromixer and method for rapid mixing of small volume liquid solutions for microfluidic bioanalytical devices and systems.
2. Description of the Related Art
Microfluidic devices that perform various chip-based chemical and biological analyses have received significant attention over the past decade. As device scales decrease below 1 mm or 500 micron, miniaturization and integration of traditional chemical and biochemical laboratory analysis devices onto credit card-sized “Lab-on-a-Chip” systems offer the potential for higher throughput by way of parallelization, shorter analysis times, reduced sample volumes, in situ operation, and reduced operation and manufacturing costs. The “Lab-on-a-Chip” technology is anticipated to have a significant impact on the fields of genomics, proteomics, clinical analysis and basic biomolecular research.
Such miniaturization and integration require careful design considerations. One important consideration is the rapid, homogeneous mixing of biological and biochemical solutions or reagents that often have relatively low diffusion coefficients. Rapid mixing is crucial in microfluidic systems for biochemistry analysis, drug delivery, sequencing or synthesis of nucleic acids, among others.
Rapid homogeneous mixing becomes increasingly important when the time scale associated with mixing is larger or of the same order as a chemical reaction time scale. Rapid mixing is difficult or inefficient in microfluidic devices and systems due to the characteristically low Reynolds number (Re) of microflows and the relatively low diffusion coefficients of the solutions to be stirred, particularly if the solutions contain macromolecules. In other words, the low Reynolds number (on the order of 0.1) associated with microfluidic devices precludes turbulence as a viable stirring mechanism. Since rapid stirring is essential for many biochemical assays and bioanalytical techniques, such as immunoassays and hybridization analyses, this presents a significant challenge to chip-based molecular diagnostics.
In small scale devices, low Reynolds number (Re) flow fields can result in mixing processes on the order of tens of seconds or greater. This is particularly true of solution streams containing macromolecules (e.g., globular proteins) whose diffusion coefficients are 1–2 orders of magnitude lower than that of most liquids. For example, the diffusive transport of hemoglobin across a 200 μm buffer stream can be on the order of 400 s.
Various micromixing schemes and devices have been developed in the art. Note “mixer” and “micromixer” are used interchangeably herein to refer to a micro-scale mixing apparatus. Similarly, “fabrication” and “microfabrication” are used interchangeably herein to refer to micro-scale fabrication. In general, a solution is mixed or homogeneous once gradients in the concentration have been eliminated. The fluid flow associated with micro-bioanalytical system is typically dominated by viscous forces and the fluid flow is therefore laminar.
In laminar flow, the reduction of gradients in concentration is usually dominated by simple molecular diffusion. Consequently, present micromixers rely on diffusion as the main mechanism for mixing. The diffusion time (tD) dependence on diffusion length (LD) can be approximated by the well known relationship,
            t      D        ≈                  L        D        2            D        ,where D is the diffusion coefficient.
A reduction in the mixing time for a solution of a given diffusion coefficient requires a reduction in the diffusion length. Accordingly, rapid micromixers are typically designed to stretch materials lines (boundaries) between two streams and to decrease the length over which diffusion occurs.
There are several known mixing schemes in the art that offer diffusion length reduction, including lamination mixing, micro-plume injection, pressure-driven chaotic advection, and parallel/serial mixing.
Lamination mixing essentially offers a technique for increasing the interfacial area between the liquids to be mixed as well as reducing diffusion length by sequentially splitting and stacking fluid layers. Although this offers an effective mixing method, the lamination mixer comprises a three-dimensional (3D) geometry, which presents costly microfabrication challenges. Lamination mixers also require significant flow channel area in order to have enough cycles. Lamination mixers are discussed in further details with respect to out-of-plane mixers hereinafter. Exemplary teachings on lamination mixing method and device can be found in “Microfluidic Devices for Electrokinetically Parallel and Serial Mixing”, Anal. Chem. 1999, 71, 4455–4459, by Jacobson et al. and in U.S. Pat. No. 6,213,151 B1, titled “MICROFLUIDIC CIRCUIT DESIGNS FOR PERFORMING FLUIDIC MANIPULATIONS THAT REDUCE THE NUMBER OF PUMPING SOURCES AND FLUID RESERVOIRS”, issued to Jacobson et al. of Knoxville, Tenn., and assigned to UT-Battelle of Oak Ridge, Tenn., USA (hereinafter referred to as “Jacobson et al.”).
Microplume injection reduces the diffusion length required to mix by injecting fluid stream A into stream B through a large array of micronozzles. The fluid emanates from the micronozzles in the form of microplumes that slowly disperse throughout the fluid. Microplume injection like lamination mixing has inherent disadvantages due to the complexity of their microfabrication. The homogeneity of the mixture is proportional to the area density of the micronozzles. A grid of micronozzles with a very fine pitch poses obvious microfabrication difficulties. Exemplary teachings on microplume injection can be found in “Towards integrated microliquid handling systems”, J. MicroMech. Microeng. 1994, 4, 227–245, by Elwenspoek et al. (hereinafter referred to as “Elwenspoek et al.”).
Chaotic mixing through the use of forcing jets has been suggested and simulated for microfluidic systems. Another scheme that has been co-developed by co-inventor J.G. Santiago involves a method for mixing two streams that takes advantage of chaotic advection. However, in order to take such advantage, a complex three dimensional (3D) geometry is required to create the complex advection flow. The resulting mixer thus requires a more complex fabrication scheme, which is a common problem with many prior art mixers. Related exemplary teachings can be found in “Chaotic Mixing in Electrokinetically and Pressure Driven Micro Flows”, Proc. 14th IEEE Workshop MEMS 2001, 483–486, by Lee et al. (hereinafter referred to as “Lee et al.”) and “Passive Mixing in a Three-Dimensional Serpentine Microchannel”, J. Microelectromech. Syst. 2000, 9, 190–197, by Liu et al. (hereinafter referred to as “Liu et al.”).
The problem of space associated with lamination mixers is also a problem for the simple parallel/serial mixing method and devices such as those disclosed by Jacobson et al. Such mixers require rather long channels in order to allow for sufficient diffusion of the solution. The size or footprint of the device is therefore a major design hurdle. Large footprints defeat the purposes of miniaturization and portability.
Micromixing devices that utilize these mixing schemes will be discussed next. Generally, most micromixers can be classified by their respective underlying mixing scheme as either active or passive. Passive stirring schemes include previously discussed simple in-plane, lamination, and chaotic advection stirring. Passive mixers typically use channel geometry to increase the interfacial area between the liquids to be mixed. These mixers can be categorized into two subclasses: in-plane mixers, which divide and mix streams within a fluid network confined to one level, i.e., a pattern that can be projected onto a single plane, and out-of-plane or lamination mixers, which use three-dimensional channel geometries.
The simplest in-plane micromixers merge two fluid streams into a single channel and accomplish mixing by molecular diffusion. More elaborate in-plane micromixers include those disclosed by Jacobson et al. and by Koch et al. in “Two Simple Micromixers Based on Silicon”, J. Micromech. Microeng. 1998, 8, 123–126.
Out-of-plane, lamination mixers can sequentially split and stack fluid streams in a three-dimensional fluidic network. Exemplary teachings can be found in “A modular Microfluid System with an Integrated Micromixer,” J. Micromech. Microeng. 1996, 6, 99–102, by Schwessinger et al., and in U.S. Pat. No. 6,241,379 B1, titled “MICROMIXER HAVING A MIXING CHAMBER FOR MIXING TWO LIQUIDS THROUGH THE USE OF LAMINAR FLOW”, issued to Larsen of Holte, DK, and assigned to Danfoss A/S of Nordborg, DE. Such mixers can achieve exponential growth of stream to stream interfacial area for multiple split-and-stack cycles. Microplume array injection disclosed by Elwenspoek et al. is another out-of-plane mixer.
The third type of passive mixer is a chaotic advection micromixer which takes advantage of rapid stretching and folding of material lines associated with pressure-driven chaotic advection, such as one disclosed by Liu et al. Lamination mixers typically need multilayer microfabrication techniques, which make them less attractive to bioanalysis system designers. This is particularly true for electrokinetic systems where one- or perhaps two-layer fabrication is the norm.
Active mixers typically have moving parts or externally applied forcing functions such as pressure or electric field. A few active micromixers have been demonstrated. One is presented by Liepmann et al. in “Micro-Fluidic Mixer”, Polym. Mater. Sci. Eng. Proc. ACS Div. Polym. Mater. Sci. Eng. 1997, 76, 549–550, where a mixing chamber is designed to effect fluid stirring using microfabricated valves and phase-change liquid micropumps. Another active mixer currently being developed by Lee et al. is a field-driven, silicon microfabricated mixer that takes advantage of dielectrophoresis to stir material in the mixer. Pressure disturbances have also been added to microchannel flows to induce rapid stirring. Although active mixers with moving parts are effective, they are often difficult to fabricate and control and are mostly suited for silicon substrates only.
U.S. Pat. No. 6,086,243, titled “ELECTROKINETIC MICRO-FLUID MIXER”, issued to Paul et al. of Fremont, Calif., and assigned to Sandia Corp. of Albuquerque, N. Mex., disclosed a method and apparatus for efficiently and rapidly mixing liquids in a system operating in the creeping flow regime such as would be encountered in capillary-based systems, those systems in which the thickness of the system is small compared to its width. According to Paul et al., by applying an electric field to each liquid, the mixer is capable of mixing together fluid streams in capillary-based systems, where mechanical or turbulent stirring cannot be used, to produce a homogeneous liquid.
Specifically, a static electric field is applied to each liquid, thereby inducing electroosmotic flow in each, the liquids being in contact with one another. By appropriately choosing the value of the static electric field, each liquid can be induced to create a zone of recirculation, thereby stirring the liquid and creating interfacial area to promote molecular mixing.
There is a continuing need in the art for a more efficient micromixer that takes advantage of an efficient low Reynolds number stirring mechanism. What is also needed is a novel mixing mechanism that be easily implemented into an active micromixer with smaller footprint, fewer components, and without moving parts. The novel mixing mechanism takes advantage of fluctuating electric fields to effect rapid mixing efficiently and effectively with easy integration and low fabrication cost, thereby enabling Lab-on-a-Chip bioanalytical microfluidic devices and systems.